Mold composition and process



UV U

MOLD COMPOSITION AND PROCESS Herbert Greenewald, In, Moorestown, N. I.

No Drawing. Application January 18, 1952, Serial No. 267,208

7 Claims. (Cl. 106-688) The present invention relates to mold materials which are particularly suited for precision casting and to processes of making molds.

A purpose of the invention is to produce a harder and stronger mold for precision casting.

A further purpose is to obtain a mold material which is not reactive with the oxide slags formed in casting high melting point alloys containing aluminum and/or titanium as alloying elements, such as Inconel X having the composition set forth below.

A further purpose is to reduce the difiiculty of forming a molding material for precision casting.

A further purpose is to increase the refractoriness of molding materials having fine surface quality and high tolerance.

A further purpose is to obtain a molding materia which is less sensitive to variations in preheating temperature and pouring temperature, and therefore which is lessdlikely to cause rejections when good control is not use A further purpose is to provide a mold material which is sufficiently strong and resistant to abrasion to permit removal of a pattern by a sliding motion.

A further purpose is to obtain a precision casting material which can be used in thin sections because of its great strength.

Further purposes appear in the specification and in the claims.

In the precision casting practice, an investment or other similar molding material is formed into a mold around a pattern, which may be of a destructible material such as wax or plastic, or may be of metal. The character of the mold has formed a limitation in the process.

In the most common prior art procedure, to obtain metal castings having a good finish and close tolerance, the wax, plastic or metal pattern is placed in a flask and a slurry forming the investment is allowed to set around the pattern until it becomes hard and the pattern is melted or burned out leaving the finished mold. This process is subject to the great economic disadvantages that a new pattern must be used for each casting. In accordance with the procedure developed by the "present inventor, a permanent pattern may be used.

In the lost wax process there are at least five steps from the production of the master pattern to the forming of the casting, at which steps contours must be transferred from one object to another. A certain amount of loss in precision is inevitable in each one of these steps, and, therefore, especially when great care is not used, inaccuracy and loss of tolerance are likely to occur. In this connection it should be noted also that the removal of the destructible pattern in the lost wax process involves heating before the destructible pattern melts, which often causes disruptive expansion, followed by burning off and preheating of the investment, which necessarily changes the investment dimensions. In the process of the present invention there are at most three steps between the production of the master pattern and the making of the casting, with elimination of the disruptive effect of removal of the destructible pattern so that very much higher tolerances are possible, and high tolerances are possible with less care and skill than in the lost wax process.

In certain prior art processes it is necessary or desirable to apply a mold wash to the investment prior to casting, and use of the mold wash results in appreciable change in dimensions both due to erosion and United States Patent uneven build-up. One of the advantages of the present invention is that no mold wash is required and in fact the use of a mold wash is undesirable.

Prior art molds formed by the methods used in the ceramic industry have involved the use of clay bodies with high forming pressure and high firing temperatures. Molds have also been made by ramming sodium silicatesilica flour mixtures. Plaster of Paris molds have also been used. These have been the best of the earlier meth ods for obtaining foraminous molds from a permanent pattern. Each of these methods has certain serious disadvantages which the procedure according to the present invention lacks. The ceramic process requires heavy presses and high temperature kilns which are not required in following the procedure of the present invention. Furthermore the high firing temperatures tend to distort the molds. The plaster of Paris molds are limited to use with nonferrous metals while the molds of the present invention may be used with iron and steel and other ferrous alloys.

The sodium silicate-silica flour molds have several disadvantages, especially the poor resistance to heat shock,

the necessity for preheating the molds to approximately 500 C. before pouring metal into them, the need for furnace cooling the molds to approximately 200 C., and the low green strength.

I have previously developed a molding composition embodied in my application Serial No. 94,055, filed May 18, 1949, for Mold Composition for Precision Casting and Method of FormingMold, since issued as U. S. Patent 2,586,814, granted February 26, 1952. This prior composition offers many advantages over the prior art above referred to. It employes a silica-base material. There are, however, certain disadvantages in the silica-base material of my prior application which the present development overcomes, as noted below.

The mold composition of the present invention bakes to a harder and stronger mold than does the silicabase material of my prior patent application.

The mold material of my prior invention also is not suitable for use in casting high melting point alloys in which aluminum and/or titanium is an alloying 1ngreclient, such as Inconel X, an 'alloy of the following nominal composition:

Percent since the oxide slags present react with the silica-base material. The composition of the present invention is excellent for casting high melting point alloys in which aluminum and/or titanium is an alloying ingredient, such as lnconel X, thus making it very useful for turbine blades and other similar castings.

The silica-base mold composition of my prior invention is not as easy to form into molds as is the composition of the present invention. Thus the composition of the present invention cuts down molding time.

The composition of the present invention also is more refractory than the silica-base mold material of my prior invention. This enables me to preheat the mold to a higher temperature before the casting is poured without breaking down or fusing the mold material. This is important in casting alloys having very high melting points such as 2500 F. to 3500 F. Also due to the greater refractoriness and less reactive character of the mold composition of the present invention much less care is required in the choice of mold temperature to obtain good castings.

The mold material of the present invention, in common with the material of my prior invention, has a very fine textured surface, which gives good surface on the castings. The tolerance is excellent, being at least as good as or 0.005 inch.

The mold material of the present invention by virtue of its strength and resistance to abrasion can be used with patterns of metals or other permanent materials which are removed by sliding motion, as for example by pulling out without drag or by unscrewing a threaded portion.

The mold material of the invention by virtue of its high strength is applicable in thin wall sections in which suificient permeability can be obtained.

The mold material of the invention is applicable in the casting of both ferrous and non-ferrous metals and alloys, and can be employed in making castings which are utilized without machining or with a minimum of machining.

The mold material of the present invention comprises essentially a mixture of from 85 to 98 percent by weight of aluminous material of the character of fused alumina, fused silicious alumina or aluminous mullite containing at least 70 percent of aluminum oxide, with from 15 percent to 2 percent by weight of a swelling clay such as bentonite and with from 10 to 25 parts by weight of water added to 100 parts of the dry ingredients. The aluminous mineral will be predominantly in the form of particles through 200 mesh per linear inch, and preferably 85 percent of the aluminous mineral by weight will be in the form of particles through 200 mesh, and the balance will also preferably be in the form of particles through 100 mesh per linear inch.

The aluminous mineral may be an alumina mineral such as fused alumina, or may be an alumina mineral having silica as an impurity such as fused silicious alumina, or may be an aluminum silicate such as an aluminous mullite, provided in any case that the content of aluminum oxide is in excess of 70 percent by weight.

The aluminous mineral of the character of fused alumina, fused silicious alumina or aluminous mullite is suitably incorporated with the swelling clay such as bentonite, and the predetermined amount of water is added to make the mold composition of the invention. The mold composition is then rammed into a flask containing a pattern, after which the pattern is removed, and the mold is baked at a temperature of 450 F. or above, during which the mold undergoes an irreversible reaction and becomes very hard and strong.

The mold composition of the invention, while it may contain 85 to 98 percent of the aluminous mineral of the character of fused alumina, fused silicious alumina or aluminous mullite, will in the preferred embodiment contain 95 to 98 percent of the aluminous mineral. With the range of 85 to 98 percent by weight of the aluminous mineral of the character of fused alumina, fused silicious alumina or aluminous mullite, the swelling .clay will range between 5 percent and 2 percent by a weight. The water content for the wider range will be between and 25 parts of water by weight per 100 parts of the dry ingredients while the water content for the narrower range will be between 10 and 22 parts by weight per 100 parts of the dry ingredients.

The 10 parts of water is preferably used with mixtures containing 2 percent clay and the 25 parts of water is preferably used with mixtures containing percent clay, intermediate clay contents requiring intermediate water additions. The water content of this molding material is much more critical than the clay content, since a reduction of clay content down to 2 percent will not seriously afiect the green or the baked strength of the mold, but a reduction of the water content by a very few percent below the optimum for the clay content used will so reduce the green and the baked strength of the mold as to render it unsuitable in some cases. It is most notable that the water content of the original mold composition has a very pronounced effect on the strength of the mold after all the water has been driven off by heat. Too much water likewise decreases green strength very seriously making the material very sticky and hard to work, and causing the molds to crack during baking.

In the best embodiment of the invention, I will employ 95 percent by weight of aluminous mineral of the character of fused alumina, fused silicious alumina or aluminous mullite, 85 percent by weight of which passes through 200 mesh per linear inch and 5 percent of swelling clay such as bentonite, with 22 parts of water added to 100 parts by weight of dry ingredients.

The molding material within the composition ranges given above is thoroughly mixed dry, the appropriate amount of water is added, and the mixture is then screened through a 4 mesh per linear inch screen to completely mix the ingredients. The molding material is now ready for use and is fairly dry and quite light and flufiy. The molding compound is now rammed into a flask around a pattern to produce a green mold having excellent strength, surface and accuracy of dimension. The ramming of the mold can be done by hand, by impact, or under molding pressure, it being noted that excellent results are obtained by molding under a pressure of approximately 1000 pounds per square inch.

After ramming, the green mold has such strength that the pattern can be removed by a steady pull or by jarring if it clears, without damaging the mold and without endangering the accuracy of the casting. Furthermore the green mold is of such exceptional strength that it is feasible to withdraw a pattern having a bend or twist so that it must follow a curved or corkscrew path in departure from the mold. Furthermore such a pattern can be withdrawn along such a curved path by violent jarring action without deforming the mold. Patterns with twist or bend in them or with no draft such as gas turbine compressor blades and air foil blades can readily be withdrawn from molds of this material.

Of course the mold material of the invention can also be used with wax and plastic patterns which must be melted and burned out.

After ramming and after pattern removal, the flask is readily removed from the mold and the green mold is then placed in a suitable oven and subjected to a temperature in excess of 450 F. for a period of time which will allow the entire mold to reach that temperature or above. Of course the time required for the center of the mold to reach 450 F. will depend upon the mold thickness and upon the oven temperature. It is necessary to bake the mold at this temperature or above in order to produce a final irreversible set in the molding material and thus render it hard enough to be handled with ease subsequently.

Also after this baking the mold does not have any noticeable tendency to pick up moisture from the air even after prolonged exposure to a humid atmosphere.

It is possible to dry the molds out as fast as the water can be evaporated from the mold surface to prevent the exudation of drops of water. Baking temperatures up to 2500 F. have been used in order to speed up the drying and preheating process without ill effects. The molds may be withdrawn from the oven and used at any time after they have reached 450 F. throughout their cross section. No care need be taken to prevent cracking from heat shock during cooling or heating of the mold as one of the new and unexpected properties of the mold material of the present invention is its great resistance to heat shock.

If it is desired to pour castings having very thin sections in the mold material of the invention it is commonly considered advisable to preheat the mold to from 1000 to 2500" F. before pouring, provided of course StillCh temperatures are suitable in casting the particular a oy.

The molding material of the invention is of such a refractory nature that it is feasible to pour high melding alloys such as steel into such molds when preheated to 1500 F. or above. This refractoriness precludes the need for mold washes which are sometimes used with other materials to prevent the molten metal from burning in to the mold.

In addition to its other advantages the mold material of the invention has the advantage of being at one and the same time very strong and refractory and highly resistant to mishandling during preheat and pouring, and yet sutiiciently weak to crack under stresses set up by shrinking of the casting during cooling, so as to prevent hot tears and deformation of the casting.

The strength of the molds being very high in both the green and the baked state, the mold walls can be quite thin and thus no difiiculty will be encountered due to low permeability in the mold wall. Furthermore because of the fineness of the aluminous mineral of the character of fused alumina, fused silicious alumina or aluminous mullite used and because of the high strength of the molds it is possible to obtain casting with a very smooth surface and fine detail with a tolerance of a few thousandths of an inch.

All percentages referred to herein are by weight.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the process and composition shown, and I therefore claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention, what I claim as cw and desire to secure by Letters Patent is:

l. A mold composition of the character which is used to produce castings having close tolerance and a fine surface finish and is suitable for casting high melting point alloys containing titanium and aluminum after baking the mold composition to a temperature above 450 F., comprising essentially 85 to 98 percent by weight of particles predominantly through 200 mesh per linear inch of a siliciouamiaeraltcontaining atdeast 70 percent aluminum oxjdcby weight and selected from tmsisting'of fused alumina. fused silicious alumina and aluminous mullite, ercent by weight of swelling cla andJmmpans-tg -weighlfm 2. A mold composition for use after admixture with water and baking to a temperature above 450 F. to produce castings having close tolerance and fine surface fin- .ish, and capable of casting high melting point alloys containing titanium and aluminum, comprising essentially 85 to 98 parts by weight of particles through 200 mesh per linear inch of an aluminous mineral containing at least 70 percent by weight of aluminum oxide and selected from the class which consists of fused alumina, fused silicious alumina and aluminous mullite and 15 to 2 percent by weight of a swelling clay.

3. A mold composition of the character which is used to produce castings having close tolerance and a fine surface finish and is suitable for casting high melting point alloys containing titanium and aluminum after baking the mold composition to a temperature above 450 F, comprising essentially 85 to 98 percent by weight of particles predominantly through 200 mesh per linear inch of an aluminous mineral containing at least 70 percent by weight of aluminum oxide and selected from the class which consists of fused alumina, fused silicious alumina, and aluminous mullite, 15 to 2 percent by weight of bentonite and from 25 to parts by weight of water added to 100 parts by weight of the dry ingredients.

4. A mold composition suitable for use after admixture with water and baking to a temperature above 450 F. to produce castings having close tolerance and fine surface finish, and capable of casting high melting point alloys containing titanium and aluminum, comprising essentially 85 to 98 percent by weight of particles through 200 mesh per linear inch of an aluminous mineral containing at least percent of aluminum oxide by weight and selected from the class consisting of fused alumina, fused silicious alumina and aluminous mullite, and 15 to 2 percent of bentonite.

5. A mold composition of the character which is used to produce castings having close tolerance and fine surface finish and is suitable for casting high melting point alloys containing titanium and aluminum after baking the mold composition to a temperature above 450 F., comprising to 98 percent by weight of particles predominantly through 200 mesh per linear inch of an aluminous mineral containing at least 70 percent of aluminum oxide and selected from the class consisting of fused alumina, fused silicious alumina and aluminous mullite, 5 to 2 percent by weight of a swelling clay and from 22 to 10 parts by weight of water added to 200 parts by weight of the dry ingredients.

6. A mold composition of the character which is used to produce castings having close tolerance and a fine surface finish and is suitable for casting high melting point alloys containing titanium and aluminum after baking the mold composition to a temperature above 450 F., comprising 95 percent by weight of an aluminous mineral, 85 percent of which is in the form of particles through 200 mesh per linear inch, at least 70 per cent of which is aluminum oxide, and selected from the class consisting of fused alumina, fused silicious alumina and aluminous mullite, 5 percent by weight of a gyellinr la of the charact of entonite and 22 parts by weight of Water to parts y we] t of the dry ingredients. 7. The process of forming a mold of high strength and refractory properties and capable of obtaining close tolerance and fine finish in the casting and also capable of casting high melting point alloys containing titanium and aluminum, which comprises ramming a mixture of 98 to 85 percent of particles of aluminous mineral, 85 percent by weight of which are through 200 mesh, containing in excess of 70 percent by weight of alumina and selected from the class consisting of fused alumina, fused silicious alumina, and aluminous mullite, from 2 to 15 percent by weight of a swelling clay and 10 to 25 parts by weight of water to 100 parts by weight of the dry ingredients and heat treating the mixture at a temperature in excess of 450 F., whereby the mold material undergoes an irreversible reaction and greatly increases its strength.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,385 Hall Aug. 15, 1939 2,380,945 Collins Aug. 7, 1945 2,504,133 Kerlin Apr. 18, 1950 2,510,220 Grube June 6, 1950 2,521,839 Feagin et al. Sept. 12, 1950 2,586,814 Greenewald Feb. 26, 1952 

1. A MOLD COMPOSITION OF THE CHARACTER WHICH IS USED TO PRODUCE CASTINGS HAVING CLOSE TOLERANCE AND A FINE SURFACE FINISH AND IS SUITABLE FOR CASTING HIGH MELTING POINT ALLOYS CONTAINING TITANIUM AND ALUMINUM AFTER BAKING THE MOLD COMPOSITION TO A TEMPERATURE ABOVE 450*F., COMPRISING ESSENTIALLY 85 TO 98 PERCENT BY WEIGHT OF PARTICLES PREDOMINANTLY THROUGH 200 MESH PER LINEAR INCH OF A SILICIOUS MINERAL CONTAINING AT LEAST 70 PERCENT ALUMINUM OXIDE BY WEIGHT AND SELECTED FROM THE CLASS CONSISTING OF FUSED ALUMINA, FUSED SILICIOUS ALUMINA AND ALUMINOUS MULLITE, 15 TO 2 PERCENT BY WEIGHT OF A SWELLING CLAY, AND 25 TO 10 PARTS BY WEIGHT OF WATER ADDED TO 100 PARTS BY WEIGHT OF THE DRY INGREDIENTS. 